Direct microscopic link between atomic dynamics and macroscopic viscosity in liquids

Determine the direct link between microscopic atomic-scale dynamics in liquids and the macroscopic shear viscosity coefficient, identifying the specific microscopic processes that give rise to momentum transport and viscous response in fluid systems.

Background

The paper emphasizes that established methods for computing viscosity—such as the Green–Kubo formalism and the Einstein–Stokes relation—do not fully reveal the microscopic mechanisms responsible for momentum transport in liquids. As temperature approaches the glass transition, some of these relations break down, underscoring the need for a mechanistic, atomic-scale understanding.

The authors situate this question within the broader challenge of developing a normal-mode-based microscopic description for liquids, where instantaneous normal mode (INM) theory provides a framework despite the presence of unstable modes. Prior work linked unstable delocalized INMs to self-diffusion, leaving unsettled the role of localized unstable modes and, more generally, the microscopic origin of viscosity. The present work proposes and validates a microscopic definition of viscosity based on unstable localized INMs (ULINMs), aiming to address this previously open question.